Composite metal laminates have found wide use commercially in a variety of applications. Composite metal articles having a dissimilar core and cladding are highly desirable commercially due to the fact that the beneficial characteristics of the core and cladding may be obtained in one composite article. In a single material or alloy frequently many properties cannot be greatly modified by alloying or thermal treatments, for example, such properties as modulus of elasticity, color, density and strength in combination with high thermal or electrical conductivity. However, by forming composites apparent properties of the cladding can be generated while retaining the bulk properties of the core material. In this manner, one can often obtain greatly modified and highly desirable properties over the single alloy at a reasonable cost.
For example, copper has the advantage of high conductivity and cold formability. By forming copper composites one can retain these desirable properties while generating properties of the cladding, such as wear resistance, color, oxidation or tarnish resistance and fine finishing, i.e., surface quality. Various uses to which composite copper articles may be put include high conductivity, high strength springs, highly efficient electrical contactors, and hardware involving extensive cold forming.
Particular among current commercial uses is composite copper coinage using cupronickel clad copper wherein the core material is copper and the cladding is cupronickel. As is well known, this laminate utilizes a core material which differs markedly in color from the material used in the cladding. It would be highly desirable to provide a copper laminate which does not suffer from this disadvantage. For example, such a laminate has been used on a dollar coin referred to as the Susan B. Anthony (SBA) dollar. The SBA dollar is similar in construction to the previous coinage laminates. Objections have been raised to the SBA dollar on the grounds of its similarity to the quarter. This objection may be overcome if one could provide a coinage material for use in the SBA dollar which was of a gold-simulated color so as to contrast with other coinage materials.
However, the preparation of composite copper materials meeting the criteria required for coinage materials presents numerous difficulties. Firstly, there is the problem of preparing a composite laminate in a simple and convenient manner so as to insure a well bonded laminate. Particularly among the problems which are encountered is to metallurgically bond copper materials which are characterized by vastly different work hardening characteristics and to obtain a well bonded laminate which is not characterized by surface waves or breaks. Furthermore, coinage requirements are severe with regard to materials and characteristics of the composite.
Several methods have been proposed for providing metallurgically bonded composites, and several materials have been proposed for this use. For example, U.S. Pat. Nos. 2,691,815, 2,753,623, 2,767,467 and others are characterized by providing a poorly bonded laminate or a green bond followed by annealing treatments to improve the metallurgical bond. These methods are generally objectionable due to the increased cost of the procedure. Also, the subsequent diffusion annealing process tends to degrade the properties of the composite. Several materials have been proposed for coinage applications, such as U.S. Pat. No. 3,753,669 which proposes outer cladding layers of coin silver or cupronickel metallurgically bonded to opposite sides of a core layer of nickel silver to provide a uniform silver color along the edges of the laminates, or U.S. Pat. No. 3,750,253 which utilizes a core layer of low carbon steel, a relatively thin layer of nickel silver metallurgically bonded to each side of the core layer, and a relatively very thin layer of nickel metallurgically bonded to the outer surface of each of the layers of nickel silver. These laminates are silver in color and do not have the highly desirable gold or simulated gold coloring. Also, the latter '253 patent represents a complicated multi-composite material.
Several procedures have been proposed for preparing metal laminates utilizing a single high reduction rolling pass which results in a well bonded metal laminate. Typical among these are U.S. Pat. Nos. 3,496,625, 3,496,621 and 3,381,366, for example. In accordance with these processes, the coper base alloy core only is heated to an elevated temperature followed by rolling together the heated core and the cold cladding at a high speed of at least 100 feet per minute in one pass at a specified high reduction with the core and cladding preferably coming together for the first time in the bite of the rolls and with the cladding contacting the roll prior to contacting the core. Alternatively, the components may be rolled together in direct face-to-face contact in one pass at a high reduction with the reduction being sufficient to cause one of the components only to recrystallize with the recrystallizaton occurring either at the bite of the rolls or immediately upon exit from the rolls. These methods are effective in providing well bonded metal laminates in a convenient and commercially viable procedure. However, they do not provide a suggestion for a gold-simulated material which can be useful as a coinage material. Also, when applied to such material it is found that wavy or cracked surface finishes may be obtained which seriously detracts from the usefulness of the procedure.
Monolithic copper alloys are known having a desirable gold color, such as U.S. Pat. Nos. 3,259,491, 3,341,369 and 3,475,227; however, these materials are not suitable for coinage and preparation of composites therefrom which are suitable presents serious problems, such as objectionable surface waviness.
Accordingly, it is an object of the present invention to provide a novel composite material and a novel method for obtaining composite materials.
It is a still further object of the present invention to provide such a material and method which is useful in making coins and which has a gold-simulated color.
It is a still further object of the present invention to provide a method and article as aforesaid which is inexpensive and wherein the resultant article has a smooth surface finish.
Further objects and advantages of the present invention will appear hereinbelow.